I changed the site overall background color by inserting this code in the quick css: body {background: #f3f3f3;}

It worked, with the following exceptions:

Custom Menu widget, on the right hand side of interior pages. The widgets displays a menu (locations) made under Menus just for this widget. The background color there remains white, it doesn’t take the site background color.

UPDATE: I guess I just solved this modifying this id on the quick css: #nav_menu-2 a {background: #f3f3f3;}